Purifying zinc metal



March 12, 1935. P, MCL, GINDER Er AL 1,994,352

PURIFYING ZINC METAL (METHOD) Filed Dec. 28, 1933 2 Sheets-Sheet 1' I I BY ATTCRNEYS Patented Mar. l2, 1 935 Nl'lED STATES PURIFYING ZINC METAL (METHOD) Philip McLean Ginder,

Willis McGerald Peirce,

and Robert Kerr Waring, Palmerton, Pa., assignors to The New Jersey Zinc Company, New York, N. Y., a. corporation of New Jersey Application December 28, 1933, Serial No. 704,244

'7 Claims.

This invention relates to purifying zinc metal by distillation, and has for its object the provision of an improved method of and apparatus for purifying zinc metal containing both lead and cadmium, or other metal impurities of higher and 1 lower boiling points than zinc.

In our copending application Serial No. 620,634, filed July-2, 1932, we have disclosed a method of and apparatus for removing cadmium and the like from zinc metal by fractional distillation in a rectifying column. In that application we have also disclosed a novel and advantageous combination of our method of cadmium elimination with the method of eliminating lead from zinc metal by the rectifying action of a reflux column described in the copending application of Messrs. Holstein & Ginder, Serial No. 540,566, filed May 28, 1931. I Our copending application Serial No. 668,314, filed April 28, 1933 likewise involves a further disclosure of this conjoint application of reflux and rectification treatments for purifying zinc metal of both lead and cadmium (or the like) in two successive treatment operations; that is to say, subjecting vapor derived from impure zinc metal contaminated with lead and cadmium to two successive reflux and rectification treatments, in the first of which a vapor purified of lead is obtained, and delivered to the second rectification treatment from which mo ten zinc metal purified of cadmium is obtained.

The present invention relates to a modification and improvement of the method and apparatus of the above-mentioned application Serial Nos. 668,314 and 620,634; said improvement consisting in delivering the metal from the first reflux or rectification treatment to the second rectification treatment in molten form instead of vapor form. The introduction of molten metal to the cadmium-eliminating rectifying column requires that a larger amount of metal be reboiled than if a vapor feed be used, but has the important advantage that less back-pressure is created in the column. Furthermore, the pressure created in the cadmium-eliminating rectifying column does not add to the pressure in the' lead-eliminating rectifying column and distillation retort, as is the case with a vapor feed. Since it frequently .happens that the limiting factor in the capacity of the combined system is the back-pressure, it is advantageous in such cases to use a liquid feed to the cadmium-elim nating rectifying column if the maximum capacity is desired.

In accordance with the practice of the present invention, a condenser is provided between the lead-eliminating rectifying column and the cadmium-eliminating rectifying column, in which condenser the lead-free vapor delivered by the lead-eliminating rectifying column is condensed to liquid metal which is then transferred to the cadmium-eliminating rectifying column. Reflux in the form of molten metal for the lead-eliminating rectifying column may be provided by con densation of vapor at the top of said column; or, an appropriate portion of the molten metal condensed in the condenser between the lead-eliminating rectifying'column and the cadmium-eliminating rectifying column may be fed back to the lead-eliminating column to serve as reflux therein. In this second case, the condenser serves not only as a means for supplying liquid feed to the cadmium-eliminating rectifying column, but also as a dephlegmator for the lead-eliminating rectifying column.

These and other features of the invention will be better understood if reference is made to the accompanying drawings, taken in conjunction with the following description, in which:

Fig. 1 is a sectional elevation of an apparatus illustrative of a practice of the invention;

Fig. 2 is a sectional elevation in part of a modi fied form of apparatus likewise illustrative of a practice of the invention; and

Fig. 3 is a sectional elevation of a modified form of rectifying column.

The apparatus shown comprises a zinc distillation retort 10 for volatilizing impure zinc metal, mounted in an appropriate heating furnace of conventional construction (not shown). A conduit 11 connects the retort with a charging well (not shown). The retort is also provided with a tapping hole of conventional type (not shown) for removing molten zinc rich in iron and/or lead, when necessary.

An elbow pipe 12 connects the retort with the bottom of a lead-eliminating rectifying column 13. A series of superposed rectangular trays or pots 14 of silicon carbide or other appropriate refractory material are employed to build up the rectifying column. The top and bottom edges of the trays are preferably beveled, as at 15, so that they may the more readily be stacked one above the other in alignment to form the column.

A transverse slot or opening 16 is located in the I are stacked in staggered relationship with respect to the slots so that they form a column of supering vapor and descending molten metal.

The built-up lead-eliminating rectifying column is supported in any appropriate manner, and provided with suitable means for inhibiting loss of heat therefrom. In the apparatus shown in Figs. 1 and 2, said means consists of appropriate heat-insulating material 18, such as refuse zinc oxide, held in place by a metal casing 19. A modified form of construction is shown in Fig. 3, consisting of a refractory wall 19 that defines a chamber or flue 20 adapted to receive and discharge suitable heating gases; the temperature of which may be regulated to control or inhibit dissipation of heat through the wall of the builtup column of trays.

In the apparatus shown in Fig. 1, the top of the lead-eliminating rectifying column is connected by a refractory pipe'21 to a condenser 22, which may consist for example of a vertical cylinder of refractory material. The condenser is provided at its top with a vent 23 for the escape of exhaust gas, and likewise for the release of any excessive pressure of metallic vapor that may be built up in the system; for example, on account of a temporary increase in the amount of metallic vapor discharged from the top of the leadeliminating rectifying column, above the amount that is condensed by the condenser. A passageway 24 connects the condenser at its bottom with a well 25 for collecting molten metal condensing therein. A liquid seal that prevents the entrance of air into the condenser through the passageway is established by a partition 26, which protects the portion of a bath of molten metal in the collecting well next to the passageway, and dips below the normal level of molten metal in the well. A cleanout port 27 is provided in the partition and makes possible access into the condenser and passageway. The collecting well may advantageously be left open or openable at its top 'so that molten metal may be ladled out when desired. An overflow conduit 28 connects the collecting well with a feed-trough 29. A needle valve 30 of conventional design is suitably.provided between the feed-trough and an enclosed charging well 31, connected by a liquid seal 32 with a charging-tray 33 within a cadmiumeliminating rectifying column 34.

The cadmium-eliminating rectifying column 34, as shown in both Figs. 1 and 2, is built-up of a series of superposed trays or pots 35 of the same general type as employed in the lead-eliminating rectifying column, the trays being, however, preferably of somewhat greater surface area and more numerous than in the lead-eliminating rectifying column. The lower part 36 of the cadmiumeliminating rectifying column is surrounded for an appropriate length thereof by a combustion or heating chamber 37 provided with a fuel gas burner 38 and a flue 39 for the escape of exhaust ases. A conduit 40 connects the base or lower end of the cadmium-eliminating rectifying column with a sump 41 for the collection of molten metal. The sump is provided with a tap hole 42 for the withdrawal of molten metal; and is surrounded by a combustion or heating chamber 43, provided with a fuel burner 44 and a stack 45 for the escape of exhaust gases.

The upper part 46 of the cadmium-eliminating rectifying column, above the combustion or heating chamber, is surrounded by an appropriate layer of suitable heat-insulating material 47. The relative lengths of the lower and upper parts of the cadmium-eliminating rectifying column surrounded by the combustion chamber and by the heat-insulation material, respectively, may be varied to meet different conditions encountered in practice.

A dephlegmator 48 surmounts and is in open communication with the cadmium-eliminating rectifying column. The amount of condensation in the dephelgmator is controlled by providing an appropriate amount of heat-insulation material around the dephlegmator. An inclined bafile 49 is provided in the dephlegmator, the lower edge of the baflie being located above an overflow well 50, provided with a skimming bar 51. A normally plugged tap hole 52 is located in the side wall of the dephlegmator adjacent to the well for the removal of accumulated metallic dust from the top surface of molten metal accumulating there- The top of the dephlegmator is connected bya conduit 53 to a canister 54 for condensing and collecting in the form of metallic dust any metallic vapor that escapes from the top of the dephlegmator. The canister is provided at its top with a vent 55 for the escape of any permanent gas that may enter the system and for the release of any excessive pressure that may be established therein.

Referring to the modified form of apparatus shown in Fig. 2, the top of the lead-eliminating rectifying column 13 is connected to the condenser 22 by a refractory conduit 56, provided with heavy heat-insulation 56' to decrease the loss of heat therefrom and thereby decrease the condensation of metallic vapor therein. A passageway 57, provided with a down-sloping bafiie 58. connects the top of the lead-eliminating column with the refractory conduit. The lower end of the passageway is in turn provided with a well 59 equipped with a skimming bar 60. A cleanout port 61 is provided adjacent the well for the removal of metallic dust accumulating on molten metal caught in the well.

The condenser 22 is provided at its bottom with lateral passageways 62 and 63 leading into a collecting well 64 located directly below the condenser. Partitions 65 and 66 extend.downward- 1y into the well below the normal liquid level thereof to establish liquid seals, whereby air is excluded from the condenser. Appropriately positioned cleanouts 67, 68, 69 and '70 are provided in the well. At on end of the well, towards the lead-eliminating rectifying column, a conduit 71 connects the well at the level of its bottom with a charging well 72 connectible with the upper end of the lead-eliminating rectifying column. This charging well is provided with a regulatable needle valve '73 adapted to discharge controlled amounts of molten metal into the leadeliminating rectifying column. At the end of the well 64 towards the cadmium-eliminating rectify: ing column 34, a dip well 25 and means for feeding molten metal to the cadmium-eliminating column, identical with those shown in Fig. 1, and described above, are provided.

In the operation of the apparatus impure'metallic zinc contaminated with impurities such as lead and iron with boiling points higher than that of zinc and cadmium with a boiling point lower than that of zinc is introduced into the distillation retort 10 through the charging conduit 11. The retort is heated at such a temperature as to volatilize the metal at an appropriate rate. The metal to be refined is charged into the retort 10 either from time to timeor continuously, so as to maintain an approximately uniform volume of metal in the retort. The retort is continuously operated until the lead accumlating therein has attained such a concentration that the vapor delivered from the retort is so high in lead that adequate purification of it from lead cannot be secured in the lead-eliminating column 13. When this has occurred, the residue rich in lead and other metals such as iron with boiling points higher than that of zinc is tapped out of the retort, and the retort recharged.

The zinc vapor contaminated with lead and cadmium evolved from the retort enters the bottom of the lead-eliminating reflux or rectifying column 13, and ascends therethrough in intimate contact with refluxing molten zinc, whereby the ascending zinc vapor is freed from lead and other constituents with boiling points higher than that of zinc.

The molten zinc'that refluxes through the leadeliminating column 13 is obtained by condensation of zinc vapor that enters it. Inthe case of the apparatus shown in Fig. 1, the molten zinc required is obtained by condensation at the top ofthe column. In the case of the apparatus shown in Fig. 2 the molten zinc is supplied by the condenser 22, in a manner hereinafter described.

The zinc vapor freed from lead as it leaves the top of the lead-eliminating column 13 passes into the condenser 22.

In the case of the apparatus shown in Fig. 1, the pipe 21 leads the zinc vapor from the top of the lead-eliminating column into the condenser 22, where said zinc vapor is condensed to molten zinc freed from lead but still contaminated with cadmium. Themolten metal condensing in the condenser 22 flows through the orifice 24 into the well 25. Access of air into the condenser is prevented by the liquid seal established by the partition 26 that dips into the molten metal in the well 25. Any permanent gases that enter the condenser 22 along with the zinc vapor escape through the orifice 23. The orifice 23 also serves to release any excess pressure that may be set up in the condenser.

The metal collecting in the well 25 overflows through the conduit 28 into the well 29, whence said molten metal can be fed as desired through the needle valve 30 into the charging well 31 whence it flows into the charging tray 33 at a point intermediate the top and bottom of the cadmium-eliminating column 34 through the metal seal 32.

In the case of the apparatus shown in Fig. 2, the zinc vapor is conducted from the top of the lead-elminating column to the condenser 22 through the conduit 56 equipped with heavy heat insulation 56' whereby loss of heat from this conduit is decreased to such an extent that only a relatively small amount of molten zinc condenses therein. Any molten zinc that does condense in the conduit56 flows into the passage '7 connected with the top of the lead-eliminating column 13, where it is diverted by the sloping baflle 58 into the well 59. The well 59 is equipped with a skimmer plate 60 which prevents any blue powder carried down by the molten zinc from being ,carried into thelead-eliminating column. Said blue powder collecting on the metal in the well 59 can be removed from time to time through the clean-out port 61.

The vapor entering the condenser 22 is condensed to molten zinc contaminated with cadmium, in the same manner as in the apparatus shown in Fig. 1. However, part or all of the reflux necessary for the lead-eliminating column 13 is transferred from the collecting well 64 below the condenser 22 through the conduit '71 into the well 72. From the well 72, molten metal can be fed into the top of the lead-eliminating column 13 as desired through the regulable needle valve '73.

In both the apparatus of Fig. 1 and Fig. 2, any metal that collects in the well 25 over and above the amount that it is desired to treat in the cadmium-eliminating column 34 can be removed from the well 25 by ladling.

The cadmium-eliminating column 34 in the apparatus of bothFigs. 1 and 2.is operated as follows:

The firing of the combustion chambers 37 and 43 is so adjusted and regulated as to volatilize either entirely ,or in part the molten mixture of zinc and cadmium entering the cadmium-eliminating rectifying column by overflowing the tray 33. The vapor thus produced is rectified as it ascends the column, so that the ascending vapor is progressively enriched in cadmium and the descending (refluxing) molten metal is progressively freed of cadmium. Cadmium-rich zinc vapor escapes from the top of the column into the dephlegmator- 48, where it is at least in part condensed to molten metal which gathers in and overflows from the well 50. Such cadmium-rich zinc vapor as is not condensed in the dephlegmator passes over into the canister 54 where it is condensed to form a cadmium-rich zinc dust. Refined zinc metal (that is, zinc metal with greatly decreased cadmium content) is tapped from sump 41 through the tap hole 42 at appropriate intervals.

The proportion of molten metal charged into the cadmium-eliminating rectifying column that must be revolatilized depends upon the composition ofthe feed, the refined zinc to be obtained, and the high cadmium fraction, as well as on the number, type and size of trays comprising the column. All but a cadmium-rich fraction of the vapor entering the dephlegmator 48 is condensed therein and is returned to the cadmium-eliminating rectifying column 34 for the further removal of cadmium therefrom.

The apparatus illustrated in the drawings as adapted for the practice of the invention may be variously modified in construction and arrange ment of operating units without departing from the principles of the invention. Various others forms of transverse baflies may be used in place of the trays illustrated in the drawings. For simplicity of description, a single operating unit is shown in the drawings, whereas in commercial practice, it will sometimes be desirable to embody in the apparatus a plurality or a battery of distillation retorts with associated lead-eliminating rectifying columns conjunction with one or more condensers and cadmium-eliminating rectifying columns in order to approximate substantial continuity of operation and uniformity of purified zinc product.

Throughout this specification and the appended claims, lead and cadmium are taken as representative of metal impurities of higher and lower boiling points, respectively, than zinc, and it is therefore to be understood that specific mention of lead and cadmium in the specification and claims is illustrative and not restrictive.

We have herein described the redistillation of impure zinc metal as the most usual source of impure zinc vapor for the practice of the invention. It will be understood however that zinc vapor containing lead and cadmium may be derived in various other ways and from other sources, as for example, directly from a zinc smelting or reducing operation.

We claim: I

l. The method of purifying zinc contaminated with lead and cadmium which comprises passing the impure zinc in the form of vapor into a leadeliminating rectifying column in which the vapor is brought into intimate contact with refluxing molten zinc, withdrawing lead-rich molten zinc from the bottom of the lead-eliminating rectifying column, withdrawing zinc'vapor freed of lead but still contaminated with the cadmium from the top of the lead-eliminating rectifying column and condensing the same to molten zinc contaminated with cadmium at the same time removing permanent gases associated; therewith, passing said molten zinc contaminated with cadmium at least in part through a liquid seal into a cadmium-eliminating rectifying column where said molten zinc is subjected to rectification, withdrawing cadmium-rich zinc from the top of the cadmium-eliminating rectifying column, and withdrawing molten zinc substantially free from lead and cadmium from the bottom of the cadmium-eliminating rectifying column.

2. The method of purifying zinc contaminated with lead and cadmium which comprises passing the impure zinc in the form of vapor into a leadeliminating rectifying column in which the vapor is brought into intimate contact with refluxing molten zinc, withdrawing lead-rich molten zinc from the bottom of the lead-eliminating rectifying column, withdrawing zinc vapor freed of lead but still contaminated with cadmium from the top of the lead-eliminating rectifying column and condensing the same, refluxing a portion of the resulting molten zinc containing cadmium through the lead-eliminating rectifying column, passing another portion of said resulting molten zinc containing cadmium into a cadmium-eliminating rectifying column where said zinc is subjected to rectification, withdrawing cadmium-rich zinc from the top of the cadmium-eliminating rectifying column, and withdrawing molten zinc substantially free from lead and cadmium from the bottom of the cadmium-eliminating rectifying column.

3. A method of purifying zinc contaminated with lead and cadmium according to claim 2 in which a portion of the lead free molten zinc is introduced into said cadmium-eliminating rectifying column intermediate its top and bottom, the lower part of said cadmium-eliminating column being appropriately heated to effect the distillation of molten zinc so introduced.

4. A method of purifying zinc contaminated with lead and cadmium according to claim 2 in which the lead-free molten zinc is passed through a liquid seal into the cadmium-eliminating column.

5. In the method of purifying zinc contaminated with lead and cadmium by subjecting the zinc vapor to a first rectification in which lead is removed and to a successive rectification in which cadmium is removed, the steps which comprise condensing the lead free zinc vapor obtained as a product of the first rectification tomolten zinc in an appropriate condenser, releasing permanent gases associated with the lead free zinc vapor from the condenser and from the equipment in which the first rectification is conducted in order to prevent excessive pressure within said condenser and said rectification equipment, and subjecting the molten zinc to rectification to remove cadmium therefrom.

6. The method of purifying zinc contaminated with lead and cadmium which comprises subjecting the impure zinc in the form of vapor to rectification to obtain lead-free zinc vapor, con densing the resulting zinc vapor to molten zinc substantially freed of lead but still contaminated with cadmium and simultaneously removing permanent gases associated with said lead-free zinc vapor, passing the resulting molten zinc through a liquid seal into a cadmium eliminating rectify ing column wherein said molten zinc is subjected to distillation and rectification to remove the cadmium, and recovering molten zinc substantially free from lead and cadmium.

'7. The method of purifying zinc contaminated with lead and cadmium which comprises subjecting the impure zinc in the form of vapor to rectification to obtain lead-free zinc vapor, condensing the resulting zinc vapor to molten zinc substantially freed of lead but still contaminated with cadmium and simultaneously removing permanent gases associated with said lead-free zinc vapor, passing the resulting molten zinc continuously into a cadmium eliminating rectifying column wherein said molten zinc is subjected to distillation and rectification to remove the cadmium and recovering molten zinc substantially free from lead and cadmium.

PHILIP McLEAN GINDER. WILLIS McGERALD PEIRCE. ROBERT KERR WARING. 

